Adeno-associated viral (AAV) vectors have been used widely for therapeutic gene transfer to different cell types in vitro and organs in vivo (Grimm et al. (2003) Curr. Gene Ther. 3:281-304). The broad tissue tropisms exhibited by different AAV serotypes can be attributed, at least in part, to the distribution of primary and secondary receptors on various cell types (Di Pasquale et al., (2003) Nat. Med. 9:1306-12; Kaludov et al., (2001) J. Virol. 75:6884-93; Qing et al., (1999) Nat. Med. 5:71-7; Summerford et al., (1999) Nat. Med. 5:78-82; Summerford et al., (1998) J. Virol. 72:1438-45). In this regard, initial cell surface binding of the viral capsid is often mediated through complex cell surface glycosaminoglycans (Grimm et al., (2003) Curr. Gene. Ther. 3:281-304). For example, AAV serotype 2 has been shown to utilize heparan sulfate (HS) as a primary receptor for cell attachment (Summerford et al., (1998) J. Virol. 72:1438-45). AAV serotypes 4 and 5, which display different tropism with respect to AAV2, utilize sialic acid with different linkage specificities for cell surface binding and transduction (Kaludov et al., (2001) J. Virol. 75:6884-93). Similarly, transduction by AAV1, which does not bind heparin, was inhibited by removal of cell surface sialic acid with sialidase (Chen et al., (2005) Hum. Gene Ther. 16:235-47). On the other hand, AAV6 which differs from AAV1 by only six amino acid residues (Rutledge et al., (1998) J. Virol. 72:309-19) and shares ˜85% homology with the AAV2 capsid sequence, binds heparan sulfate and can be purified using heparin-affinity chromatography (Halbert et al., (2001) J. Virol. 75:6615-24). It is interesting to note that the AAV6 capsid does not possess the R585 and R588 residues that are primarily responsible for HS binding by AAV2 (Kern et al., (2003) J. Virol. 77:11072-81; Opie et al., (2003) J. Virol. 77:6995-7006).
Purification schemes for AAV2 based on heparin affinity purification are well defined. Less streamlined are the purification parameters for other serotypes, which do not bind to heparin. It would be desirable to engineer heparin-binding properties into other AAV capsids to provide vectors with universal purification attributes for greater ease of purification. Further, it would be advantageous to transfer AAV capsid sequences conferring desirable characteristics (e.g., tropism) to other AAV capsids to engineer virus vectors with an array of improved properties.